The present invention relates to circuit breakers including molded case circuit breakers and earth leakage breakers, in particular to structures of current interrupting sections in the circuit breakers.
This type of circuit breakers as mentioned above is a double-break type circuit breaker that comprises first and second fixed contactors of a current interrupting section in parallel arrangement, a bridge type rotatably movable contactor that holds movable contacts on the ends of a U-shaped arm, opposing fixed contacts attached on the ends of the fixed contactors, and a grid type arc extinguishing device which is disposed opposing and in front of the movable contactor (for example Japanese Patent Publication (Kokai) No. 11-273536).
FIG. 8 and FIG. 9 show a conventional structure of the double-break type circuit breaker and its current interrupting section. Referring to FIG. 8, the circuit breaker comprises a main body casing 10 of the circuit breaker, a handle 11 for switching operation, a switching mechanism 12, and a current interrupting section 13. As described afterwards, the current interrupting section 13 is composed of a pair of fixed contactors arranged in parallel and connected to the line side and the load side terminals, a bridge type movable contactor opposing these fixed contactors and linking to the switching mechanism, and an arc extinguishing device.
FIG. 9 shows a detailed structure of the current interrupting section, wherein the reference numeral 1 designates a first fixed contactor extending to the line side terminal; 2 is a second fixed contactor extending to the load side terminal; 1a and 2a are fixed contacts attached on the ends of the fixed contactors 1 and 2; numeral 3 is a rotatably movable bridge type contactor having movable contacts 3a and 3b opposing the fixed contacts 1a and 2a and attached on the ends of a U-shaped arm; 4 is an arc extinguishing device disposed in front of the movable contactor 3 and along the path of switching movement of the movable contactor 3; 4a shows grids (magnetic plates) arranged vertically between side walls of the arc extinguishing device 4; and numeral 5 is a rotating shaft of the movable contactor 3, the movable contactor 3 linking to the switching mechanism 12 (FIG. 8) through a contactor holder.
When the movable contactor 3 in this structure is opened, as shown in FIG. 10 in the event of current interruption, as is well known, arc “arc” develops between the fixed contacts 1a, 2a of the first and second fixed contactors 1, 2 and the movable contacts 3a, 3b of the movable contactor 3. The arc extends from I to II in FIG. 10 by an electromagnetic driving force and is divided by the grids 4a of the arc extinguishing device 4 to raise the arc voltage and receive a cooling effect. Thus, the arc is extinguished and the electric current is limited and interrupted.
The current interrupting section of double-break type as described above, however, has a problem in that abnormal dissipation of the fixed and movable contacts occurs with repeated current interruption as described below, causing poor contact performance between the fixed and movable contacts in a closed state. A mechanism in the abnormal dissipation of contacts will be described in FIGS. 11(a)-11(c).
The current interrupting section of double-break type as shown in FIG. 9 generates arc 1 and arc 2 between the fixed contacts 1a, 2a and the movable contacts 3a, 3b in which electric currents i of the arcs are in reversed directions with each other as shown in FIG. 11(a). As a result, a repulsive electromagnetic force F acts on the arc 1 generated between the fixed contact 1a and the movable contact 3a and the arc 2 generated between the fixed contact 2a and the movable contact 3b to force the arcs to separate each other. Therefore, beginning points of the arcs on the fixed and movable contacts move from the center of the contact surface outwardly in the opposite directions due to the repulsive electromagnetic force F as shown in FIG. 11(b).
Interruption of a heavy short circuit current in the current interruption section generally causes dissipation of the movable contacts 3a and 3b due to melting and evaporation in the surface region of the contacts by energy of the arc. When the beginning points of the arc 1 and arc 2 move away on the contact surface outwardly as shown in FIG. 11(b), dissipation of the contacts is concentrated at the end regions as shown in FIG. 11(c). This unbalanced dissipation causes poor contact performance (decrease of contact area) between the fixed and movable contacts in the closed state of the contactor mechanism, which in turn leads to troubles such as extraordinary heating and adhesion of the contacts in the current flowing condition.
Therefore, an object of the present invention is to provide an improved circuit breaker of a double-break type that suppresses the abnormal dissipation of contacts as shown in FIG. 11(c) to achieve a long life, avoid the poor contact performance between contacts in a closed state, and improve arc extinguishing capability.
Further objects and advantages of the invention will be apparent from the following description of the invention.